From nobody Sun Dec 05 20:53:33 2021
X-Original-To: freebsd-net@mlmmj.nyi.freebsd.org
Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2610:1c1:1:606c::19:1])
	by mlmmj.nyi.freebsd.org (Postfix) with ESMTP id 9E3D118BDE03
	for <freebsd-net@mlmmj.nyi.freebsd.org>; Sun,  5 Dec 2021 20:53:46 +0000 (UTC)
	(envelope-from lutz@iks-jena.de)
Received: from annwfn.iks-jena.de (annwfn.iks-jena.de [IPv6:2001:4bd8::19])
	(using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits))
	(Client did not present a certificate)
	by mx1.freebsd.org (Postfix) with ESMTPS id 4J6f162NgXz3Fbj;
	Sun,  5 Dec 2021 20:53:46 +0000 (UTC)
	(envelope-from lutz@iks-jena.de)
X-SMTP-Sender: IPv6:2001:4bd8:0:666:248:54ff:fe12:ee3f
Received: from belenus.iks-jena.de (belenus.iks-jena.de [IPv6:2001:4bd8:0:666:248:54ff:fe12:ee3f])
	by annwfn.iks-jena.de (8.15.2/8.15.2) with ESMTPS id 1B5KrYwZ031569
	(version=TLSv1 cipher=DHE-RSA-AES256-SHA bits=256 verify=NOT);
	Sun, 5 Dec 2021 21:53:34 +0100
X-MSA-Host: belenus.iks-jena.de
Received: (from lutz@localhost)
	by belenus.iks-jena.de (8.14.3/8.14.1/Submit) id 1B5KrXKk008967;
	Sun, 5 Dec 2021 21:53:33 +0100
Date: Sun, 5 Dec 2021 21:53:33 +0100
From: Lutz Donnerhacke <lutz@donnerhacke.de>
To: John Hay <jhay@meraka.org.za>
Cc: "Bjoern A. Zeeb" <bz@freebsd.org>, Kurt Jaeger <pi@freebsd.org>,
        freebsd-net <freebsd-net@freebsd.org>
Subject: Re: why multi-hop icmp redirects to 0.0.0.0 on 13.0 ?
Message-ID: <20211205205333.GC8367@belenus.iks-jena.de>
References: <Yast8d1dRd7ZFWr3@fc.opsec.eu> <alpine.BSF.2.00.2112041609520.68830@ai.fobar.qr> <YauV138P1xyB53Ja@fc.opsec.eu> <alpine.BSF.2.00.2112041633090.68830@ai.fobar.qr> <YaufYWCM9YOUG5Op@fc.opsec.eu> <alpine.BSF.2.00.2112041905270.68830@ai.fobar.qr> <alpine.BSF.2.00.2112051718171.68830@ai.fobar.qr> <CAGv8uarUvftonP9LBOPmVksqTz7pFuQoUP2OJFG4BKBTex6VAw@mail.gmail.com>
List-Id: Networking and TCP/IP with FreeBSD <freebsd-net.freebsd.org>
List-Archive: https://lists.freebsd.org/archives/freebsd-net
List-Help: <mailto:freebsd-net+help@freebsd.org>
List-Post: <mailto:freebsd-net@freebsd.org>
List-Subscribe: <mailto:freebsd-net+subscribe@freebsd.org>
List-Unsubscribe: <mailto:freebsd-net+unsubscribe@freebsd.org>
Sender: owner-freebsd-net@freebsd.org
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAGv8uarUvftonP9LBOPmVksqTz7pFuQoUP2OJFG4BKBTex6VAw@mail.gmail.com>
X-message-flag: Please send plain text messages only. Thank you.
User-Agent: Mutt/1.5.17 (2007-11-01)
X-Rspamd-Queue-Id: 4J6f162NgXz3Fbj
X-Spamd-Bar: ----
Authentication-Results: mx1.freebsd.org;
	none
X-Spamd-Result: default: False [-4.00 / 15.00];
	 REPLY(-4.00)[]
X-Spam: Yes
X-ThisMailContainsUnwantedMimeParts: N

On Sun, Dec 05, 2021 at 08:20:08PM +0200, John Hay wrote:
> Something I have observed is that if you use FreeBSD 13 as a router with 2
> subnets on the same interface, it will generate redirects when hosts send
> packets to the other subnet via the FreeBSD router. I think it is wrong.

No, it's correct.

> The host does not have a more direct way to get to the other subnet.

The other host can arp for an address in a non-connected network on the
interface because it's the same L2 domain. Hence the ICMP redirect is send
out to provide the shortcut (skipping the router).

> RFC792
> on page 13 does not talk about interfaces, but networks, "If G2 and the
> host identified by the internet source address of the datagram are on the
> same network...".

"network" == "layer 2 domain".